The present invention relates to a coextruded, biaxially oriented multilayer film suitable for use as an electrically insulating film. The multilayer film is particularly useful as a dielectric for the production of electrical film capacitors.
The use of plastic films as electrical insulation materials has been known for a long time. These films are used not only as dielectrics in capacitors but also for cable insulation or as self-adhesive insulating tapes. The electrical properties of films of this type must satisfy very onerous requirements. Not only must the dielectric loss factor be low and the breakdown voltage be high but these characteristic values must be retained over a long period of time, especially at elevated temperatures.
A number of publications deals with biaxially oriented polypropylene films, which are said to have a particularly high breakdown voltage. A good breakdown voltage is mostly obtained by using polypropylene raw materials which have quite specific properties.
Thus, EP-A-0,011,796 describes the use of a biaxially oriented polypropylene film as electrically insulating film which has two layers and is used in metallized form for producing capacitors. The raw material for this electrically insulating film is extremely pure polypropylene which has a residual ash content of less than 100 ppm, is free from organic or inorganic lubricant additives and contains no ionogenic components.
EP-A-0,222,296 discloses a coextruded, biaxially oriented multilayer film in which the base layer likewise comprises extremely pure raw material for electrical films, based on polypropylene polymers. In order to improve the electrical properties of the film when used as a dielectric in capacitors, the stabilizer content of the outer layer or layers is significantly higher than the stabilizer content of the base layer. In connection with electrically insulating plastic films, stabilizers are understood to mean specific compounds which reduce the occurrence of oxidation and thus the time dependence of the dielectric loss factor and electrical breakdown voltage, when these films are used in capacitors as dielectrics in combination with thin layers of metal acting as electrodes.